BGP - How to influence partial network on outgoing update

[guibarati]

Hi,

Does anyone know if it's possible to influence only a portion of the networks updates been sent over eBGP connection?

If I am advertising the network 100.1.1.0/24, and I have this network directly connected on one interface, the router is connected to two ISPs (with different AS's). Can I influence the updates to say the host 100.1.1.20 has better path through ISP A and everything else ISP B?

[cadet alain]

Hi,

Routing is destination based not source based if you want  to do source based routing then you need to use PBR(Policy Based routing).

Regards

Alain

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[guibarati]

Hi,

Thanks!

But I don't want to do source based I want to say:

Internet: when you want 100.1.1.0/24 come through ISP A

             when you want 100.1.1.20/32 come through ISP B

I know it has implications on the internet routing table and everything. But I want to know if it's possible.

[cadet alain]

Hi,

You should be able to send a /32 if you've got a /24 with BGP advertise-map and BGP exist-map and so advertise this /32 to the ISP you want the return traffic to pass through and send the /24 only to the other.

I'm gonna lab it up and give you the config if I can make it work.

Regards

Alain

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[guibarati]

Thank you Cadet Alain,

I still don't see how it would be possible. But if you can get it to work, please let me know.

[guibarati]

Ok, I found an easier way.

if I have 100.1.1.0/24 on fa 0/0 for exemple.

!!!!!!!!!!!!!!!!

int fa0/0

no shut

ip address 100.1.1.1 255.255.255.0

ip route 100.1.1.20 255.255.255.255 fasteth0/0

router bgp 100

network 100.1.1.0 mask 255.255.255.0

network 100.1.1.20 mask 255.255.255.255

!!!!!!!!!!!!!!!!!

Thanks anyway!

[Joseph Nelson]

Just curious,

Is the fa0/0 interface facing the ISP or your internel network? If fa0/0 is facing the ISP, it seems at best a routing loop would occur.

[Joseph Nelson]

I believe the IETF standard is a minimum /24. Some ISPs allow /24 to /28 but no greater than /28.

All the ISPs I've worked *with* or worked for never excepted anything greater thatn /28?

How then can you advertise a /32 as you propose?

[cadet alain]

Hi,

if ISPs filter prefixes > /28 then it's gonna problematic indeed, good to state this info because I had completely forgot this fact.

Regards

Alain

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[guibarati]

Thanks for the input from both.

This was a lab scenario for a possible real implementation.

If they accept /28 it works to me. I just wanted to know how to do it and used /32 as exemple.

I can just change the /32 local route to /28 and then change attributes for the /28 network only if I need to.

[Joseph Nelson]

Guibarati,

If your goal is influence inbound routing from the Internet, I'd suggest you have a look at your ISPs existing Traffic Engineering (TE) mechanisms.

As Alain alluded to you, can you can use a route-map to match on the prefixes you want to TE and then apply the ISP's TE mechanism. For example, an ISP may want you to put a specific community on to suppress routes or pad your AS_PATH.

Can I ask what is your goal in trying to TE for a given prefix? Are you trying to reduce inbound load on one of your edge routers? If so, figuring out the traffic volume per AS for that prefix will go a long way. For example, if AS701 sends you x amount of traffic, you should TE for AS701 using the ISPs TE mechanism.

HTH

[guibarati]

I'm going to install a site with 3x 100Mbps links.

So I wanted to do something to do traffic share and to be able to change attributes to only part of the network.

If I had "network 100.1.1.0 mask 255.255.255.0" for exemple, I couldn't match "'100.1.1.0/25" on the route-map for expemple.

I want, if possible, to influence inbound and outbound traffic.

I know I would need PBR for source based, and was working on the inbound traffic.

Thanks you!

[cadet alain]

Hi Joseph,

yes you're right and my suggestion of an advertise-map was irrelevant as both the /24 and the /28 would have to exist in the BGP RIB table so they would be advertised to both anyway so your suggestion of using traffic engineering techniques such as AS-PATH prepending or communities is the right way to go to influence traffic destined to a certain advertised prefix.

Regards

Alain

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[guibarati]

Yep, that was my first concern, how to influence AS-PATH or communities for only part of the network I was advertising.

The only way I could think was adding a route with bigger prefix to the local interface.

[Joseph Nelson]

You can only TE for prefixes you announce. If you don't announce a more-specific to the ISP, then you can only TE for the aggregate ( the large prefix that you actually announce).

You should note that any TE you do to influence traffic within the Internet can be short lived. So you should target the set of ASes that actually send you traffic.